Operation of an integrated circuit (IC) depends on the electrical propertie of materials of which the circuit is formed. Therefore, by monitoring selected electrical properties of these materials in the course of an IC fabrication, an effective control over the manufacturing process can be accomplished. Since modern ICs are typically formed in the shallow region below the surface of a semiconductor wafer, critical electrical properties are directly or indirectly related to the condition of the surface region of the semiconductor wafer. Surface properties of a semiconductor wafer can be monitored during IC fabrication through in-line monitoring procedures such as described in U.S. Pat. No. 4,544,887 and U.S. Pat. No. 5,661,408.
The in-line monitoring procedure permits surface properties of a semiconductor wafer to be measured without contacting the wafer surface, so that the surface properties are not affected by the measurement itself. However, unlike conventional methods of electrical characterization in which contaminant ions and molecules from the ambient environment do not significantly affect the measurements due to formation of a permanent contact on the surface of the wafer, non-contact measurements may not provide accurate information about the wafer. This is because contaminant ions and molecules from the ambient environment may adsorb on the surface of the wafer during transportation and storage, changing surface properties of the wafer. Consequently, it may be difficult to obtain accurate information about the surface properties of the semiconductors with the in-line monitoring procedures unless the semiconductors re surface treated prior to being measured.
According to one known method, a p-type silicon wafer is surface treated y being immersed in dilute hydrofluoric acid, rinsed with deionized water and dried in order to restore an inversion condition. A width of surface depletion layer measured under an inversion condition provides net doping concentration in the sub-surface region. Existing surface preparation steps, however, are typically not integrated with subsequent measurement steps. In addition, existing processing and monitoring methods are not capable of establishing the surface condition of a wafer by means of a simple treatment in ambient environment at atmospheric pressure as needed to carry out a specific electrical measurement.
Therefore, there is a need for an apparatus and method which determine selected electrical parameters of semiconductor wafers representing the outcome of individual processing steps, independent of the ambient atmosphere in which the wafers are stored or transported.